1. Field of the Invention
The present invention relates to a propeller unit that is connected to a propeller shaft of a vessel propulsion apparatus, to a shock absorber that is installed in the propeller unit, and to the vessel propulsion apparatus that includes the propeller unit.
2. Description of the Related Art
A propeller unit of a vessel propulsion apparatus includes an outer hub that includes a plurality of blades provided on an outer peripheral surface of the outer hub, an inner hub that is coaxially connected to an inner side of the outer hub, a bushing that is connected to a propeller shaft, and a damper that is disposed between the bushing and the inner hub and that is made of an elastic material such as rubber. A driving force transmitted from the propeller shaft is transmitted to the outer hub through the bushing, the damper, and the inner hub. As a result, the blades disposed at the outer hub rotate, and thrust is transmitted to a vessel.
There is a case in which an outboard motor that is one example of the vessel propulsion apparatus includes a shift mechanism by which a propulsion direction is switched between a forward direction and a backward direction. This shift mechanism includes a dog clutch, a forward gear, and a backward gear. The propeller shaft is spline-coupled with the dog clutch and the dog clutch is arranged so as to selectively engage the forward gear and the backward gear. The rotation of a drive shaft that transmits a driving force of an engine is always transmitted to the forward gear and to the backward gear. The forward gear and the backward gear are rotated in mutually opposite directions by a driving force from the drive shaft. When the dog clutch engages the forward gear, the blades disposed at the outer hub rotate in a direction in which the vessel is propelled forwardly. When the dog clutch engages the backward gear, the blades rotate in a direction in which the vessel is propelled backwardly. When the dog clutch is in a neutral state in which the dog clutch engages neither the forward gear nor the backward gear, a driving force of the engine is not transmitted to the blades.
When the dog clutch moves from a neutral position to an engaging position, an impact sound and vibrations are generated. These impact sound and vibrations result from a speed difference in the rotational direction between the forward or backward gear and the propeller shaft, and result from a rotational portion including the propeller having a great inertial mass. The neutral position is a position in which the dog clutch engages neither the forward gear nor the backward gear. The engaging position is a position in which the dog clutch engages either the forward gear or the backward gear. The rotational portion includes the propeller and a portion that rotates together with the propeller.
Particularly in a state in which the propeller unit is not rotating and the dog clutch is moved from the neutral position to the engaging position, a great shock occurs, and a great sound is generated. Likewise, at a shift-out time at which the dog clutch is moved from the engaging position to the neutral position, a great shock occurs, and a great sound is generated, although the shock at the shift-out time is smaller than a shock at the shift-in time at which the dog clutch is moved from the neutral position to the engaging position.
To lessen these shocks and sounds, a propeller unit has been proposed in which a damper made of an elastic material such as rubber is disposed between a bushing and an inner hub as described above.
For example, U.S. Pat. No. 5,049,034 discloses a propeller unit in which a shock absorber is disposed between a bushing and an inner hub. This shock absorber includes a plurality of cylindrical members each of which is made of an elastic material. Each cylindrical member is disposed between an outer peripheral surface of the bushing and an inner peripheral surface of the inner hub. In this state, each cylindrical member is disposed between an outer projecting rib provided at the outer peripheral surface of the bushing and an inner projecting rib provided at the inner peripheral surface of the inner hub.
U.S. Pat. No. 6,471,481 and U.S. Pat. No. 6,685,432 also disclose a propeller unit in which a shock absorber is disposed between a bushing and an inner hub. This shock absorber includes a cylindrical member made of an elastic material. This cylindrical member is disposed between a bushing and an inner hub. The cylindrical member includes a key way that extends from one end portion of the cylindrical member in an axial direction and a key way that extends from the other end portion of the cylindrical member in the axial direction. The key way extending from the one end portion of the cylindrical member and the key way extending from the other end portion of the cylindrical member are arranged alternately in a circumferential direction of the cylindrical member. The outer projecting rib provided at the outer peripheral surface of the bushing is fitted to the key way extending from the one end portion of the cylindrical member, whereas the inner projecting rib provided at the inner peripheral surface of the inner hub is fitted to the key way extending from the other end portion of the cylindrical member.
U.S. Pat. No. 6,659,818 also discloses a propeller unit in which a shock absorber is disposed between a bushing and an inner hub. This shock absorber is made of an elastic material. The shock absorber includes a plurality of cylindrical members arranged in parallel with each other at predetermined intervals therebetween, a first connection member by which one end portion of one of the cylindrical members is connected to one end of portion another cylindrical member in an alternate manner, and a second connection member by which one opposite end portion of the one cylindrical member is connected to one opposite end portion of the another cylindrical member in an alternate manner. Each cylindrical member is disposed between the bushing and the inner hub. In this state, each cylindrical member is disposed between a rib provided at an outer peripheral surface of the bushing and a rib provided at an inner peripheral surface of the inner hub.
U.S. Pat. No. 7,223,073 discloses a propeller unit in which a shock absorber is disposed between a bushing and an inner hub. This shock absorber is made of an elastic material. The shock absorber includes a pair of cylindrical members and a belt-shaped member by which the pair of cylindrical members are connected to each other. Each cylindrical member is disposed between an outer peripheral surface of the bushing and an inner peripheral surface of the inner hub. The cylindrical member is furthermore disposed between an outer projecting rib provided on the bushing and an inner projecting rib provided on the inner hub. The belt-shaped member is disposed between the outer projecting rib of the bushing and the inner peripheral surface of the inner hub.
In every one of the above propeller units, the shock absorber is interposed between the outer projecting rib of the bushing and the inner projecting rib of the inner hub, and thus a shock is lessened, and an impact sound is lowered when the bushing rotates relatively to the inner hub.
In every one of the above propeller units, it is possible to lessen a shock or a sound generated when the dog clutch shifts in the forward gear or the backward gear and when the dog clutch shifts out from the forward gear or from the backward gear. However, a shock lessening effect and an impact sound reducing effect have not necessarily been satisfactory.
Additionally, there is a case in which the dog clutch generates vibrations or a sound at a time excluding the shift-in time or the shift-out time, i.e., the dog clutch generates vibrations or a sound even when the dog clutch is in engagement the forward gear or the backward gear. This results from the fact that the rotation of a drive shaft driven by an engine pulsates and the fact that the pulsations of the rotating drive shaft are transmitted to the propeller shaft.
In other words, a rotational portion including the propeller, i.e., the propeller and a portion rotating together with the propeller have a great inertial mass, and thus attempt to rotate at a constant speed. Therefore, the dog clutch that is spline-coupled with the propeller shaft also attempts to rotate at a constant speed. On the other hand, the rotation of the forward gear and the backward gear engaging the dog clutch pulsates in accordance with the pulsations of the rotating drive shaft. Therefore, there is a case in which the pulsations of the rotating drive shaft cause vibrations or sounds between the dog clutch and a gear engaging the dog clutch. Vibrations and sounds generated in a state in which the dog clutch engages the forward gear or the backward gear in this way occasionally give users uncomfortable feelings particularly when a surrounding environment is quiet. These vibrations and sounds have not been satisfactorily prevented or eliminated in each of the above propeller units.